Question

At which part of the respiratory system does oxygen enter the bloodstream? In the bronchial tube, the alveolus, the heart, or the trachea?

Answer 1

Oxygen will enter circulatory system from alveoli.

Explanation:

Air is breathed in through external nares and it travels along 'respiratory tree':- at the end of a highly branched respiratory tract air enters alveoli. Wall of alveoli is thin, surrounded by capillary network across which diffusion of gases can take place.